The present invention is directed generally to a ribbon mask device, and in particular to a ribbon mask disposed between the tip end of a wire dot-matrix print head in a serial printer and a platen for separating a sheet of print paper from an ink ribbon in the printer.
FIGS. 5, 6 and 7 of the drawings depict a ribbon mask such as that disclosed in co-pending U.S. patent application Ser. No. 407,859, filed Aug. 13, 1982, now U.S. Pat. No. 4,492,484 and assigned to the same assignees as the present application. In a serial printer having a wire dot-matrix print head 21, a thin ribbon mask 26 is disposed in a small clearance (hereinafter referred to as a "platen gap") between a tip end 21-2 of print head 21 and a platen 24 for separating a sheet of print paper 27 and an ink ribbon 28 to thereby prevent problems which would occur due to contact therebetween. The platen gap, which is set to be constant, is highly important for sufficient performance of the print head and for achieving good printing quality.
To meet these requirements, the print head is assembled so that print wires 23, as they are in a standby position, will lie flush with tip end 21-2 of print head 21, and thereafter tip end 21-2 and print wires 23 are simultaneously ground to provide the platen gap, indicated at E. One method which has been widely employed in the art to achieve the desired platen gap has been to assemble and adjust the parts while a thickness gauge is placed between tip end 21-2 and platen 24 before ribbon mask 26 is assembled in place.
The position where ribbon mask 26 is situated in platen gap E more or less affects the performance of travel of the print paper and the ink ribbon. In order to allow ink ribbon 28 to run stably between tip end 21-2 and ribbon mask 26, print head 21 has on its tip end 21-2 small projections 21-3 and 21-4 which are slightly higher than the thickness of the ink ribbon to thereby provide a clearance or gap F. A gap G is also provided to permit print paper 27 to travel stably, and is automatically maintained by platen gap E, gap F, and the thickness of ribbon mask 26. Gaps F and G, as well as platen gap E, play quite important roles in the printing device.
Because of the small projections 21-3 and 21-4 on the tip end of the print head as disclosed in application serial no. 407,859, however, a grinding stone, as it grinds the tip end 21-2 of the print head, tends to have a fixed area with which the tip end 21-2 is ground, and hence becomes locally worn. Therefore, the grinding stone has to be shaped to contour many times. In addition, a thickness gauge cannot easily be placed in position upon adjustment of the platen gap.
Ribbon mask 26 has an opening 26-5 for allowing print wires 23 to press ink ribbon 28 against print paper 27. If ribbon mask 26 is displaced in the directions of arrows 32 or 33, then print wires 23 will hit ink ribbon 28 at the edges of opening 26-5, resulting in a reduced printing capability of print head 21. As a consequence, printed characters on other marks will become light in color, or sometimes wires 23 will hit ribbon mask 26, failing to effect printing.
Since ribbon mask 26 is displaced in the direction of arrow 34 when attaching and removing ink ribbon 28, there is a clearance or gap H provided between each of hooked portions 26-3 and 26-4 of ribbon mask 26 and a corresponding one of holders 25. Therefore, ribbon mask 26 is liable to be displaced in the direction of arrow 32, with the result that printed characters or marks will be light in color or no printing can be effected at certain times. Furthermore, when ink ribbon 28 is damaged due to prolonged use and catches ribbon mask 26, ribbon mask 26 is likely to disengage from holders 25 since the hooked portions 26-3 and 26-4 are small, and hence will be deformed. Accordingly, an improved ribbon mask which overcomes the above-noted problems, is desired.